Dental prosthetic devices—such as crowns, bridges, inlays, onlays, and veneers—are preferably formed from materials having good mechanical properties, including high flexural strength and high fracture toughness. In addition, these materials should preferably have good aesthetic characteristics, meaning that they have a natural appearance in color, texture, translucency, and shape so that they are not readily distinguishable from the original natural teeth.
Ceramic materials, and particularly yttria-stabilized zirconia (YSZ), have been widely adopted for use in dental restorations because these materials have high strength and high fracture toughness. For example, U.S. Pat. No. 9,309,157 (assigned to Tosoh Corporation) describes zirconia sintered bodies for use in dental applications that are formed from zirconia powder containing 2-4 mol % yttria as a stabilizer, and having reported three-point bending strength of 1,000 MPa or higher.
Conventional methods of manufacturing dental ceramic materials include molding a mixture of starting materials that includes yttria-stabilized zirconia powder into a green body, typically by press molding methods such as uniaxial pressing or cold isostatic pressing (CIP). In addition to the yttria-stabilized zirconia powder, the starting materials for the conventional press molding methods typically include a small amount (e.g., from 0-0.25 wt % of the zirconia powder) of aluminum oxide (Al2O3) as an additive, and an organic binder (e.g., from 0.5 to 10 wt %) to facilitate the press molding process. The green body can then be sintered at a sintering temperature of from 1350 to 1600° C. to obtain a zirconia sintered body.
Recent attention has been paid to increasing the concentration of yttria in zirconia powder starting materials used in dental ceramics in order to improve the translucency of the resulting sintered bodies. For example, US 2016/0310245 (also assigned to Tosoh Corporation) describes zirconia sintered bodies for use in dental applications that are formed from zirconia powder containing 4-6.5 mol % yttria as a stabilizer. The '245 publication describes sintered bodies having total light transmittance of 37-40% to light with a wavelength of 600 nm, however, the increased yttria concentration also has the result of decreasing the reported three-point bending strength to less than 900 MPa. In particular, in the examples having yttria concentrations of more than 4.1 mol % the sintered bodies had three-point bending strengths of less than 800 MPa.
The International Organization for Standardization (ISO) specification of requirements for ceramic materials used in dentistry—ISO 6872:2015—requires that materials used for monolithic ceramic prostheses of up to 3 units should have a flexural strength of at least 500 MPa. Additionally, ISO 6872:2015 requires that materials used for monolithic ceramic prostheses of 4 or more units should have a flexural strength of at least 800 MPa. Accordingly, it would be desirable to have dental ceramic materials and manufacturing methods that provide dental restorations having high translucency while still maintaining flexural strengths of greater than 500 MPa, or greater than 800 MPa.